Drug addiction is typically characterized by two features, a craving or compulsion to take the drug and an inability to limit intake of the drug. Additionally, drug dependence is associated with tolerance, which is the loss of effect of the drug with repeated administration and withdrawal, defined as the appearance of physical and behavioral symptoms when the drug is not consumed following chronic use. Sensitization occurs if the repeated administration of a drug leads to an increased response to each dose. Tolerance, sensitization, and withdrawal are phenomena reflecting some sort of plastic change occurring in the central nervous system in response to continued use of a substance. Drug addiction is related to allostasis, i.e. a resetting of the homeostatic set point or reference around which normal fluctuations occur.
Researchers in the field of drug dependence and reward have identified neurological substrates involved in animal motivation and reward and how the neural mechanisms of these substrates are engaged to result in an addictive state (George F. Koob, “Drugs of abuse: anatomy, pharmacology and function of reward pathways,” TiPS—May 1992 [Vol. 13]). The mesolimbic dopamine system which innervates the nucleus accumbens has been determined to be the portion of the brain which plays a critical role in mediation of the reinforcing aspects of addiction and the reinforcing aspects of withdrawal. In 1954, Olds and Milner demonstrated the reward circuits of the brain by electrical stimulation of the septal area of the brain (“Positive reinforcement produced by electrical stimulation of septal area and other regions of rat brain,” J. Comp. Physiol. Psychology 47:419-427, 1954).
Additionally, electrical stimulation of nervous tissue of patients has been used to treat addiction and its symptoms. Transcutaneous nerve stimulation has been proposed as a means of relieving the symptoms of withdrawal from an addictive substance (see, for example, U.S. Pat. Nos. 3,946,745, 4,841,973, 4,865,048, 5,458,625, and 5,593,432). Transcranial electrical fields have been applied to the brain (U.S. Pat. No. 4,646,744) to depolarize nerve cells as a means of treating addictions. The effects of transcranial electrical stimulating fields on withdrawal from addictive substances has been enhanced by the coadministration of a neuroactive chemical promoter (U.S. Pat. No. 5,084,007).
Deep brain stimulation has also been suggested to treat addiction in patients. For example, U.S. Pat. No. 6,109,269 discloses stimulation of multiple sites in the brain to treat addiction in a patient. In the method of U.S. Pat. No. 6,109,269, electrical stimulation using a continuous train of electrical pulses can be employed to excite or inhibit neural activity at a particular stimulation site. For example, high frequency stimulation can be employed to override or block neural activity associated with addiction in a manner similar to the use of high frequency stimulation in Parkinson's Disease to override or block neural activity associated with tremor. U.S. Pat. No. 6,109,269 also discloses combining deep brain stimulation with infusion of various pharmaceutical agents to treat addiction.